Temperature measurement technique for high-temperature gases using a tunable diode laser.

The recent development of tunable ir diode lasers suggests a new technique for the measurement of temperature in high-temperature gases. The proposed technique involves rapidly tuning the laser wavelength to measure the relative absorption in two nearly coincident vibration-rotation lines originating from different vibrational levels of the same iractive species. In this paper, we discuss the two-line tech­ nique and present results from a feasibility study in which temperature was determined in a laboratory flame using ab­ sorption by CO in the fundamental band near 4.7 μm. Although the specific requirements for a temperature measurement depend on the application, there is general in­ terest in nonperturbing in situ techniques capable of oper­ ating over a wide range of elevated temperatures. Currently, such temperature measurements are important in combustion research. In many applications the ability to temporally re­ solve temperature fluctuations also is critical. Tunable diode laser absorption spectroscopy has the potential to meet these requirements whenever a line-of-sight measurement is suit­ able, and ir-active species are present. This study represents an extension of recent work to mea­ sure absorption lineshapes in the postflame region of a flat flame burner. As in the previous work, the diode laser was rapidly modulated in wavelength across a narrow spectral interval. However, such intervals have now been chosen to encompass two nearly coincident absorption lines originating from different vibrational levels. The laser transmittance vs wavelength is recorded on an oscilloscope, and the relative absorption at the two line centers is used to determine the temperature. Although this study apparently is the first application of tunable diode lasers to temperature measure­ ments, the concept of determining temperature by two-line laser absorption has been discussed previously. The theory of the two-line absorption method is straight­ forward. Consider first the absorption in an isolated line,